Piezoelectric fuel injectors are used in vehicles to control the amount of fuel injected into the cylinders of an internal combustion engine, such as a diesel engine. The amount of fuel injected depends on the size of the orifice of a nozzle within the injector, and this, in turn, is controlled by a valve needle which moves in relation to a valve seating by an amount which depends on the voltage across a piezoelectric actuator.
An electric current is supplied to the piezoelectric actuator which stores the charge and develops a corresponding voltage across its terminals which is directly proportional to the quantity of charge stored.
Examples of such piezoelectric fuel injectors are described in EP 0995901 A and EP 1174615 A. In such injectors the nozzle needle is opened by the energy supplied to the piezoelectric actuator and the needle lift is a function of the electrical energy supplied. At high fuel pressures, a relatively large force is required to lift the valve needle from its seating, but once the needle is lifted by a certain amount, fuel pressure builds up under the valve needle and the force required to lift the needle any further diminishes rapidly, so that the needle is caused to lift extremely quickly. While fast needle opening is desirable for low-smoke emission, excessive speed causes difficulty in control of the fuelling delivered by the injector. The injector of EP 1174615 A partly addresses this problem by providing a two-stage motion amplifier, but at high pressure there are still some fuelling situations where accurate control is critical but not necessarily possible.
FIG. 1(a) shows a series of typical voltage (or charge) vs. time waveforms (voltage/charge-time waveforms) for an injector of the type described in EP 1174615. Voltage/charge-time waveform 1 illustrates the minimum voltage required to cause an injection and voltage/charge-time waveform 2 illustrates the waveform required to lift the injector needle and hold it at full lift for a period of time. FIG. 1(a) also shows representative negative-gradient slopes (dashed lines) illustrating cases where the fuel injection is terminated prior to the maximum voltage/charge level. The slope 3a of the voltage/charge-time waveform 1, 2 is proportional to the current flow to or from the actuator. Note that the injectors of EP 0995901 A and EP 1174615 A are of the “de-energize to inject” type, i.e. a voltage is reduced to start an injection, but the voltage/charge-time waveforms have been inverted here as an aid to understanding.
FIG. 1(b) shows corresponding fuel quantity delivered vs. time graphs (fuel delivery curves) for an aged injector (curve 9) and for an injector in a new condition (curve 4). As the actuator ages its piezoelectric activity diminishes and, as the nozzle seat wears, its effective area changes (increasing or decreasing, depending on the design). Both of these effects can cause a shift in the voltage/charge level required to initiate an injection from an initial level 5 to an “aged” level 6. These effects are seen by comparing fuel delivery curves 4 and 9. The age/wear effects result in a change of the minimum delivery pulse time from an initial value 7 to an aged value 8, and a shifting of the gain curve from the initial fuel delivery curve 4 to the aged fuel delivery curve 9. Where the slope of the fuel delivery curve is low, the fuelling variation 10 is relatively small, but where the slope is high the fuelling variation 11 is much larger. When the injector is run in an engine, an additional effect is that coking/lacquering of the nozzle causes the flow to reduce, making the needle lift faster so that the steep part of the fuel delivery curve gets steeper, but the slope when fully lifted is lower, resulting in a new fuel delivery curve 12. Combining the aforementioned effects results in a fuel delivery curve 13, which is sometimes higher, e.g. at region 14, and sometimes lower, e.g. at region 15, than the original fuel delivery curve 4. This combined effect is extremely difficult for an engine control unit (ECU) to correct for as there is no easy way of knowing how much of each contributing effect has occurred.
The fuel delivery curve 4 for the new injector in FIG. 1 shows three distinct sections of different slope. From the charge level 5 required to initiate injection to the charge level 16 required to switch into hydraulic lift amplification, the slope of the fuel delivery curve is low. This is advantageous for accurate control of pilot injections. From the voltage/charge level 16 required to start hydraulic amplification to the voltage/charge level 17 at full needle lift, there is a steep slope section. This is because of the fast needle lift during this period caused by a combination of the hydraulic amplification and the pressure building under the nozzle seat helping to open the needle. Once full needle lift is reached the slope of the fuel delivery curve reduces again.
FIG. 2 illustrates voltage/charge drive waveforms and corresponding fuel quantity delivered vs. time graphs (fuel delivery curves) which show the effect of increasing the current supplied to the piezoelectric actuator. By increasing the current, the slope 3b of the voltage/charge-time waveform increases. This means that the change 18 in minimum delivery pulse required to start an injection, caused by the change in voltage/charge from level 5 to level 6, is reduced. This in turn reduces the variation 19 in pilot injection quantity. Because the higher current level causes the needle to open faster, however, the slope of the second region of the fuel delivery curve is increased, resulting in there still being a large variation 20 in the fuel quantity delivered in this region. As with FIG. 1(a), negative-gradient slopes are shown (dashed lines) which illustrate termination of the fuel injection prior to the maximum voltage/charge level.
The present invention seeks to provide arrangements for driving the injector where the fuelling variation can be reduced over the full range of fuel deliveries.